Recently, the development of optical elements utilizing the electrowetting effect has progressed. The electrowetting effect is a phenomenon in which, when a voltage is applied between conductive liquid and an electrode, energy at a solid-liquid interface between the surface of the electrode and the liquid changes and the shape of the liquid surface changes accordingly.
Part A of FIG. 14 and part B of FIG. 14 are principle diagrams illustrating the electrowetting effect. As shown in part A of FIG. 14, an insulating film 2 is formed on a surface of an electrode 1, and a droplet 3 of electrolyte is placed on the insulating film 2. The surface of the insulating film 2 is subjected to water repellent finishing. In a no-voltage state shown in part A of FIG. 14, interaction energy between the surface of the insulating film 2 and the droplet 3 is low and a contact angle θ0 is large. Here, the contact angle θ0 is an angle between the surface of the insulating film 2 and a tangent line of the droplet 3, and depends on characteristics such as the surface tension of the droplet 3 and the surface energy of the insulating film 2.
On the other hand, as shown in part B of FIG. 14, when a predetermined voltage is applied between the electrode 1 and the droplet 3, electrolyte ions in the droplet 3 collect at the surface of the insulating film 2, so that an amount of charge of a charge double layer changes and a change in the surface tension of the droplet 3 is induced. This phenomenon is the electrowetting effect, and the contact angle θv of the droplet 3 changes in accordance with the magnitude of the applied voltage. In other words, in part B of FIG. 14, the contact angle θv can be expressed as a function of voltage V as in the following Equation (1):
                              [                      Equation            ⁢                                                  ⁢            1                    ]                ⁢                                                                                                cos          ⁢                                          ⁢                      θ            V                          =                              cos            ⁢                                                  ⁢                          θ              0                                +                                                                      ɛ                  0                                ⁢                ɛ                                            2                ⁢                                                                  ⁢                e                ⁢                                                                  ⁢                                  γ                  LG                                                      ⁢                          V              2                                                          (        1        )                            γLG: Surface tension of electrolyte        e: Film thickness of insulating film        ∈: Relative dielectric constant of insulating film        ∈0: Magnetic permeability of vacuum        
As described above, the surface shape (curvature) of the droplet 3 changes in accordance with the magnitude of the voltage V applied between the electrode 1 and the droplet 3. Therefore, when the droplet 3 is used as a lens element, an optical element having a focal position that can be electrically controlled can be obtained.
Now, the development of optical apparatuses including optical elements structured as above is progressing. For example, a lens array for a strobe apparatus has been proposed in Japanese Unexamined Patent Application Publication No. 2000-356708. In this example, a varifocal lens is structured by enclosing conductive liquid and droplets of insulative liquid arranged in an array pattern on a water-repellent film provided on a surface of a substrate. In this structure, individual lenses are formed in the shape of the interface between the insulative liquid and the conductive liquid. The shape of each lens is electrically controlled by utilizing the electrowetting effect, and accordingly the focal length is varied.
In addition, Japanese Unexamined Patent Application Publication No. 2004-252444 discloses the structure of a display device utilizing the electrowetting effect. In this display device, cells containing colored droplets are arranged in an array pattern, and a desired color image is displayed by selectively driving the cells. It is possible to structure the above-described cells not only as an image displaying unit but also as a lens element, such as a varifocal lens. An example of such a structure is shown in part A of FIG. 15 and part B of FIG. 15. FIG. 15 shows the schematic structure of a lens array 50 structured by arranging the lens elements in an array pattern. Part A of FIG. 15 is a plan view of a common substrate 54 included in the lens array 50, and part B of FIG. 15 is a sectional view of the main part of the lens array 50.
The lens array 50 includes a plurality of lens elements 53 in which lens surfaces are formed by interfaces between first liquid 51 that is conductive and second liquid 52 that is insulative. The first liquid 51 and the second liquid 52 have refractive indices that are different from each other, and exist without being mixed with each other. The individual lens elements 53 are two-dimensionally arranged in a sealed liquid chamber formed between the transparent common substrate 54 and a transparent lid body 55. The adjacent lens elements 53 are separated from each other by separation walls 56. A transparent electrode film 58 is formed on the bottom surface of the common substrate 54, and the top surface of the common substrate 54 is subjected to water repellent finishing in an area where the second liquid 52 comes into contact therewith. A transparent electrode film 57 is formed as a counter electrode on the bottom surface of the lid body 55 in an area where the first liquid 51 comes into contact therewith.
In the lens array 50 having the above-described structure, when a voltage applied between the pair of transparent electrode films 57 and 58 is controlled, the shape of the interface between the first liquid 51 and the second liquid 52 in each lens element 53 varies. Therefore, it becomes possible to reversibly vary a focal length of light that passes through the lens array 50, and the lens array 50 is suitable for use as a varifocal lens in a strobe apparatus of a camera.
However, in the above-described lens array 50 according to a related art, the periphery of each of the lens elements 53 is surrounded by the separation walls 56. Therefore, in a process of manufacturing the lens array 50, each of the cells must be individually supplied with, in particular, the second liquid 52 of the first liquid 51 and the second liquid 52 constituting the lens elements 53. The amount of this work increases as the number of element increases. In addition, there may be a case where the supply amount differs between the cells and characteristics of the lens elements 53 become non-uniform.
The present invention has been made in view of the above-described problems, and an object of the present invention is to provide a lens array capable of making the characteristics of lens elements uniform.